Passover has always, like always, been observed on Nisan 14. Nisan 14 is the date when the first full moon occurs after the Spring Equinox, and it has been used to keep with the lunar connections of the old Jewish calanders.

Now, if you go read something like John 19:14 , you will note that Christ was crucified on or just before Passover, during preparations for it (there are contentions as to whether it was actually Passover or not, and I haven't had enough coffee to get into those details.) You really don't need to follow that link, though. Everyone knows about the Last Supper.